By way of illustration, document WO 02/097241 illustrates a blade of the first type and document FR 2 858 650 illustrates a blade of the second type.
The function of such rims is to limit the areas facing each other between the free end of the blade and the corresponding annular surface of the turbine or compressor casing so as to protect the body of the blade against damage that might be caused by coming into contact with an annular segment, while nevertheless providing sealing between the rotor and the stator.
This function of sealing the ends of blades is most important since it determines the performance of the turbomachine, concerning efficiency, both in a turbine stage and in a compressor stage having moving blades. Depending on the operating conditions of the turbomachine, various instability phenomena exist that have the effect of reducing the efficiency of a turbomachine and/or leading to mechanical or thermal overloads on the blades.
In addition, there is clearance between the tops of the blades and the stator. In operation, this situation leads overall to friction appearing between the blades and the stator which can lead to damage to the ends of the blade and/or to the surface of the stator, or to possible variation in the above-mentioned clearance. In addition, the rotor and the stator that is concentric thereabout are of shapes that are not strictly perfectly circular, and variations in differential thermal expansion as well as mechanical deformations resulting from the stresses acting thereon change the dimensions of such parts.
In order to reduce such friction while guaranteeing the necessary sealing, it is the practice to use sealing linings suitable for being worn away, which linings are constituted by rings of “abradable” materials placed on the inside surfaces of the stator where they face moving blades. This means that the sealing linings of the stator become worn or abraded as the blades move past them so as to become matched to the shapes of the blades. Under such circumstances, the blades are provided with rims forming wipers on their radially-outer peripheries, for the purpose of co-operating with abradable linings, said rims themselves presenting a variety of streamlined shapes and being made of abrasive material.
For blades of the second type, the rim is not in the form of a plurality of wipers, but in the form of a generally continuous rim defining an open cavity at the free end of the blade, but said rim nevertheless performs the same function.
Such rims are usually made by being cast simultaneously with the remainder of the blade, after which they are finished by machining in order to give them their final shape.
In addition, in order to avoid damage to or even destruction of such rims, particularly when they are located in a high pressure compressor or turbine, they can be coated by thermal spraying (plasma torch, high velocity oxyfuel (HVOF), . . . ) an abrasive deposit e.g. of the alumina and titanium dioxide or carbide type, e.g. on an underlayer of an alloy of aluminum, chromium, and nickel for providing bonding.
Deposition by thermal spraying is a technique that is expensive and requires relative spray angles to be maintained between the axis of the torch and the surfaces of the parts to be coated so that the impact of the sprayed particles is as orthogonal as possible relative to the surface for coating in order to obtain satisfactory quality and adhesion for the deposit.
Furthermore, when depositing such a coating, the propulsive or plasma-generating gases used for spraying must be capable of being removed easily but without blowing away the projected powder by creating turbulence.